Cathodes for use in electric discharge tubes



Oct. 14, 1969 F. H. R. ALMER ETAL 3,472,693

CATHODES FOR USE IN ELECTRIC DISCHARGE TUBES Filed Feb. 9, 1966 BOCO3+ SI CO3 MIXED WITH I I v 2TO4/o by WEIGHT Of 2 Ti H2 NICKEL SLEEVE INVENTORS. FRIEDRICH H. R. ALMER BY ADRIANUS KUIPER United States Patent U.S. Cl. 117-221 2 Claims ABSTRACT OF THE DISCLOSURE A thermionic cathode for an electric discharge tube having a support with an electron-emissive layer of alkaline-earth metals containing 2. to 4% by Weight of titanium hydride thereon and having a nominal operating temperature below 700 C. real temperature.

The invention relates to a cathode for use in an electric discharge tube, more particularly to a cathode the emissive layer of which consists of barium-strontiumoxides.

The usual operating temperature of such a cathode is approximately 750 C. (real temperature). This temperature is based on a compromise. At an excessively high temperature a satisfactory emission is obtained, it is true, since the reduction of the oxides of the emissive layer proceeds more rapidly, but the evaporation of Ba also increases considerably, as a result of which the life is shortened. The formation of a poorly conducting intermediate layer between the emissive oxide layer and a support consisting of active nickel is also strongly promoted so that such a layer will cause difficulties before the usual life of the cathode has been attained. In indirectly heated cathodes, the insulation between cathode and heating wire moreover materially deteriorates owing to the high temperature.

An excessively low cathode temperature on the contrary is favourable for the insulation between cathode and heating wire, and for obtaining a lower evaporation of barium, it is true, but the release of barium from the oxides is effected much more slowly, as is known in chemical reactions, so that notwithstanding the lower evaporation of barium the quantity of active barium in the layer decreases. It has consequently been found that in addition to other causes a real temperature of less than 700 C. brings about a considerable reduction of the life of the cathode.

It is known that titanium and titanium hydride may have a favourable influence on the emissive layer, but it has been found in practice that it is not desirable to simply mix these substances with the Ba-Sr oxides since at the said usual cathode temperature an intermediate layer of high resistance is then soon formed. It has therefore already been suggested to replace Ti by rhenium. Since Ti reduces the electric resistance of the oxide layer itself, it has already been suggested to use this substance in heavily loaded cathodes. In order to avoid the disadvantages of the formation of an intermediate layer of 3,472,693 Patented Oct. 14, 1969 high resistance, the Ti-containing layer was separated, however, from the Ba-Sr oxide layer by a layer consisting of pure nickel or by gauze. In this manner, the reducing action of Ti was consequently utilized without difficulties arising as a result of an intermediate layer of high resistance.

Another reason for which the presence of Ti in the emissive layer is undesirable, consists in that such a Ticontaining layer has a darker surface than a layer consisting of Ba-Sr oxide so that the heating energy must be raised in order to attain the usual cathode temperature, which is also undesirable.

It has been found that the said disadvantages can be avoided while retaining the advantages if, according to the invention, the emissive barium-strontium oxide layer of the cathode contains 2 to 4% by weight of titanium hydride while the nominal operating temperature of the cathode lies below 700 C. real temperature. Owing to the lower operating temperature the formation of an insulating intermediate layer now proceeds so slowly that this layer does not cause any difiiculties during the life of the cathode. It has further been found that the TiH prevents the life of the cathode from being shortened as a result of the low operating temperature. In the first place the TiH promotes the reduction of the emissive oxides so that a sufiicient quantity of Ba is released notwithstanding the low operating temperature, but moreover the TiH prevents this Ba from being oxidized again by oxidizing gases.

The applicant has found that the reduction of life at an excessively low cathode temperature is mainly due to the fact that oxidizing gases released from the electrodes oxidize the barium still available in the oxide layer, since these gases are not bound sufliciently rapidly owing to the lower evaporation of Ba while oxides, if any, formed by these gases in the layer are decomposed less rapidly. Owing to the fact that the operating temperature of a cathode in accordance with the invention may be considerably lower than is usual, it is not required for the heating energy to be raised although the emissive layer has a darker surface. Owing to this fact alone, the cathode temperature is reduced by C. to C. It has been found that in many cases the heating energy can be even reduced by 20 to 30% without the cathode being adversely affected.

As a result, the heating by radiation from the cathode of grids and anode present in the tube and consequently also the gas development of these electrodes are reduced, while moreover in indirectly heated cathodes the insulation between cathode and heating wire is materially improved owing to the low temperature of heating wire and cathode.

The invention is of importance especially with regard to the tendency to reduce the dimensions of discharge tubes to a minimum, since the insulating layer between cathode and heating wire may be thinner and the electrode distances may be reduced.

What is claimed is:

1. A thermionic cathode for an electric discharge tube comprising a support and an electron-emissive layer thereon, said electron-emissive layer consisting essentially of alkaline-earth oxides and 2 to 4% by weight of titanium 3 4 hydride, said cathode having a nominal operating tem- 3,159,461 12/1964 MacNair 313346 X perature below 700 C. real temperature. 3,333,141 7/1967 Lemmens et a1. 313-346 2. A thermionic cathode as claimed in claim 1 in which the support is nickel and the alkaline-earth oxides are FOREIGN PATENTS barium and strontium oxides. 5 1,103,263 5/1955 France.

References Cited WILLIAM L. JARVIS, Primary Examiner UNITED STATES PATENTS US. Cl 3,099,577 7/1963 Lemmens et a1 117-224 3,155,864 11/1964 Coppola 313-346 117224;25Z 57;313 346 73% UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3472693 Dated October 14, 1969 Inventor) FRIEDRICH HERMANN RAYMUND ALMER AND ADRIANUS KUIPER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 7, change "U. S. PHILIPS COMPANY INC." to read -U. S. PHILIPS CORPORATION-7 Signed and sealed this 1st day of Sept 1970.

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